U.S. patent number RE32,171 [Application Number 06/517,937] was granted by the patent office on 1986-06-03 for method for the manufacture of an electret fibrous filter.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to Jan van Turnhout.
United States Patent |
RE32,171 |
van Turnhout |
June 3, 1986 |
Method for the manufacture of an electret fibrous filter
Abstract
A method for the manufacture of an electrically charged fibrous
filter from a highly molecular non-polar fiber material wherein a
.[.web.]. .Iadd.film .Iaddend.of the fiber material is continuously
fed and stretched. At least one side of the stretched .[.web.].
.Iadd.film .Iaddend.is homopolarly charged by a plurality of corona
charging elements. The charged .[.web.]. .Iadd.film
.Iaddend.material is then fibrillated, collected and processed into
a filter.
Inventors: |
van Turnhout; Jan (Delft,
NL) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
|
Family
ID: |
19821033 |
Appl.
No.: |
06/517,937 |
Filed: |
August 1, 1983 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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315139 |
Oct 26, 1981 |
|
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|
|
929680 |
Jul 31, 1978 |
30782 |
|
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Reissue of: |
929680 |
Jul 31, 1978 |
RE031982 |
Oct 27, 1981 |
|
Reissue of: |
561872 |
Mar 25, 1975 |
03998916 |
Dec 21, 1976 |
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Foreign Application Priority Data
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Mar 25, 1974 [NL] |
|
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7403975 |
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Current U.S.
Class: |
96/99; 264/435;
55/528; 210/508; 307/400; 361/233; 128/205.29; 361/226; 264/DIG.47;
264/484; 264/479; 264/413 |
Current CPC
Class: |
H01G
7/023 (20130101); B29C 71/0081 (20130101); B01D
39/1623 (20130101); B01D 39/083 (20130101); B01D
39/10 (20130101); D01D 5/423 (20130101); Y10S
264/47 (20130101); Y10S 264/48 (20130101); B01D
2239/0695 (20130101); B01D 2239/10 (20130101); B01D
2239/065 (20130101); B01D 2239/0654 (20130101) |
Current International
Class: |
B29C
71/00 (20060101); B01D 39/16 (20060101); B01D
39/08 (20060101); D01D 5/42 (20060101); D01D
5/00 (20060101); H01G 7/02 (20060101); H01G
7/00 (20060101); D01D 005/42 () |
Field of
Search: |
;264/22,25,27,DIG.47,DIG.48,24 ;425/174.8E ;55/155,528 ;210/508
;128/205.29 ;307/400 ;361/226,233 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Thurlow; Jeffery
Attorney, Agent or Firm: Sell; Donald M. Smith; James A.
Truesdale; Carole
Parent Case Text
.Badd.This is a continuation reissue of application Ser. No.
315,139 filed Oct. 26, 1981, now abandoned, which is a division
reissue of Ser. No. 929,680 filed July 31, 1978, now Re. 30,782.
.Baddend.
Claims
I claim: .Badd..[..[.1. A method for the manufacture of an
electrically charged fibrous filter from a high molecular non-polar
material, comprising the steps of:
continuously feeding a film of high molecular isotactic
polypropylene material at a rate of 12.2 meters per minute;
stretching said film in two stages, said first stage of stretching
stretches said film at a ratio of 1:6 at a temperature of
approximately 110.degree. C., said second stage of stretching
stretches said film at a ratio of 1:1.5 at a temperature of
substantially 130.degree. C.;
homopolarly charging at least one side of the stretched film with a
plurality of corona charging elements, said plurality of corona
discharge elements being connected to minus 10 KV, said step of
charging including the use of a metal plate connected to an
opposite polarity voltage source and a grid whereby said charging
corona elements are between said metal plate and said film, said
metal plate being connected to a voltage of plus 3 KV, said grid
being connected to a voltage of minus 2.3 KV, said corona charging
elements being spaced substantially 5 mm from the means for
supporting said film during said second stage of stretching;
fibrillating the charged film into fiber material; .Iadd.and
.Iaddend.
collecting the fiber material .[.; and processing the collected
fiber material into a filter.]. .Iadd.to form a
filter..]..]..Iaddend. .Badd..[..[.2. A method for the manufacture
of an electrically charged fibrous filter from a high molecular
non-polar material, comprising the steps of:
continuously feeding a film of high molecular isotactic
polypropylene material at a rate of 12.2 meters per minute;
stretching said film in two stages, said first stage of stretching
stretches said film at a ratio of 1:6 at a temperature of
approximately 110.degree. C., said second stage of stretching
stretches said film at a ratio of 1:1.5 at a temperature of
substantially 130.degree. C.;
homopolarly charging at least one side of the stretched film with a
plurality of corona charging elements, said plurality of corona
discharge elements being connected to minus 3.2 KV, said step of
charging including the use of a metal plate connected to an
opposite polarity voltage source whereby said charging corona
elements are between said metal plate and said film and said metal
plate being connected to a voltage of plus 3 KV, said corona
charging elements being spaced substantially 5 mm from the means
for supporting said film during said second stage of
stretching;
fibrillating the charged film into fiber material; .Iadd.and
.Iaddend.
collecting the fiber material .[.; and processing the collected
fiber material into a filter.]. .Iadd.to form a
filter..]..]..Iaddend. .Badd..[..[.3. A method as in claim 2
wherein said step of homopolarly charging said film takes place
simultaneously with said second stage of stretching..]..]..Baddend.
.Badd..[..[.4. A method as in claim 2 wherein said step of
homopolarly charging takes place simultaneously with the second
stage of stretching at the region of highest heat application on
said moving film..]..]..Baddend. .Badd..[..[.5. A method as in
claim 2 wherein said film is bi-laterally charged by a plurality of
coronoa elements on each side of said film..]..]..Baddend.
.Badd..[..[.6. A method as in claim 2 wherein said step of
homopolarly charging includes the step of using a grid connected to
the same polarity voltage source as said plurality of charging
corona elements such that said grid is between said charging corona
elements and said film..]..]..Baddend. .Badd..[..[.7. A method as
in claim 5 wherein said step of stretching is accomplished in two
stages and said step of homopolarly charging said film takes place
simultaneously with said second stage of stretching..]..]..Baddend.
.Badd..[..[.8. A method as in claim 7 wherein said step of
homopolarly charging takes place simultaneously with the second
stage of stretching at the region of highest heat application on
said moving film..]..]..Baddend. .Badd..[..[.9. A method as in
claim 6 wherein said film is bi-laterally charged by a plurality of
corona elements on each side of said film..]..]..Baddend.
.Iadd..[..[.10. A method for the manufacture of an electrically
charged fibrous filter from a highly molecular non-polar material,
comprising the steps of:
continuously feeding a film of said material;
heating said film
stretching said film along the longitudinal axis thereof as defined
by the path of movement of said film;
homopolarly electrically charging at least one side of said
film;
fibrillating the charged film into fiber material; and
collecting the fiber material to form a filter..]..]..Iaddend.
.Iadd..[..[.11. A method as in claim 10 wherein said film is
stretched substantially nine times the original length
thereof..]..]..Iaddend. .Iadd..[..[.12. A method as in claim 10
wherein said step of stretching includes the step of heating said
film wherein the temperature is slightly below the melting
temperature of said film..]..]..Iaddend. .Iadd..[..[.13. A method
as in claim 10 wherein said step of stretching includes the step of
heating said film and the temperature is dependent on the speed at
which the film is continuously fed, said temperature increasing
with increasing speed of film feeding..]..]..Iaddend.
.Iadd..[..[.14. The method as in claim 10 wherein said step of
stretching includes the step of stretching said film in two stages
wherein both stages of stretching include the application of heat
to said film and wherein the temperature in both stages is slightly
lower than the melting temperature of said film..]..]..Iaddend.
.Iadd..[..[.15. A method as in claim 10 wherein said step of
stretching includes the step of stretching said film in two stages
wherein both stages of stretching include the application of heat
to said film and wherein the temperature of the second stage of
stretching is greater than the temperature of the first stage of
stretching..]..]..Iaddend. .Iadd..[..[.16. A method for the
manufacture of an electrically charged fibrous filter from a highly
molecular non-polar material, comprising the steps of:
continuously feeding a film of said material;
heating said film
stretching said film;
homopolarly electrically charging at least one side of said
film;
fibrillating the charged film into fiber material substantially in
the longitudinal direction thereof as defined by the path of
feeding of the film; and
collecting the fiber material to form a filter..]..]..Iaddend.
.Iadd..[..[. 7. A method as in claim 16 wherein said step of
fibrillating employs a needle roller having a higher peripheral
velocity than the speed of feeding of said film..]..]..Iaddend.
.Iadd..[..[.18. A method as in claim 16 wherein said step of
fibrillating includes spreading the film fibers, which spreading is
enhanced by the electrostatic charges injected during said step of
charging..]..]..Iaddend. .Iadd..[..[.19. A method for the
manufacture of an electrically charged fibrous filter from a highly
molecular non-polar material, comprising the steps of:
continuously feeding a film of said material;
heating said film;
stretching said film;
homopolarly electrically charging at least one side of said
film;
fibrillating the charged film into fiber material; and
collecting the fiber material on a roller and simultaneously taking
one or more layers lying on top of the other together from said
roller to form a filter..]..]..Iaddend. .Iadd..[..[.20. A method
for the manufacture of an electrically charged fibrous filter from
a highly molecular non-polar material, comprising the steps of:
continuously feeding a film of said material;
heating said film;
stretching said film;
homopolarly electrically charging at least one surface of said film
using a corona device spaced from the surface of said film whereby
the electric charge is sprayed onto the surface of said film;
fibrillating the charged material into fiber material; and
collecting the fiber material to form a filter..]..]..Iaddend.
.Iadd..[..[.21. The method as in claim 20 wherein said step of
charging coincides with said step of stretching..]..]..Iaddend.
.Iadd..[..[.22. A method as in claim 21 wherein said step of
charging includes the steps of inserting a metal grid between said
corona device and the surface of said film, and supporting said
film on a grounded support member..Iaddend..]..]. .Iadd..[..[.23. A
method as in claim 22 wherein said step of charging includes the
step of applying an alternating voltage to the corona wires of said
corona device..]..]..Iaddend. .Iadd..[..[.24. A method as in claim
22 wherein said step of charging increases the spraying intensity
of the charge onto the surface of the film by positioning a metal
plate a greater distance from the film surface than said corona
device..]..]..Iaddend. .Iadd..[..[.25. A method as in claim 20
wherein said step of homopolarly charging at least one side of said
film includes charging both surfaces of said film by respective
corona devices spaced from the respective surfaces of said
film..]..]..Iaddend. .Iadd..[..[.26. A method as in claim 25
wherein the step of charging the film surface includes the step of
applying equal but opposite charging to the respective opposite
surfaces of said film..]..]..Iaddend. .Iadd..[..[.27. A method as
in claim 20 wherein said step of charging includes ageing the film
thermally to obtain high charge persistence and also to increase
the thermal stability of the charge of said film..]..]..Iaddend.
.Iadd..[..[.28. A method as in claim 20 wherein said step of
charging includes a first step of charging one surface of the film
with one charge polarity coincidentally with said step of
stretching by a corona device spaced from said film surface, and a
second subsequent step of charging the other film surface with a
charge of opposite polarity by a second corona device spaced from
said other film surface..]..]..Iaddend. .Iadd..[..[.29. A method as
in claim 21 wherein said step of stretching includes the
application of heat to said film and said step of charging includes
positioning said at least one corona device in a region of the
highest temperature applied during said step of
stretching..]..]..Iaddend. .Iadd..[..[.30. A method for the
manufacture of an electrically charged fibrous filter from a highly
molecular non-polar material, comprising the steps of:
continuously feeding a film of said material;
stretching said film in two stages along the longitudinal axis
thereof as defined by the path of movement of said film, and
including the step of applying heat to both said two stages;
homopolarly charging said film in said second stage;
fibrillating the charged film into fiber material substantially
along said longitudinal axis; and
collecting the fiber material to form a filter..]..]..Iaddend.
.Iadd..[..[.31. A method as in claim 30 wherein said step of
fibrillating employs a needle roller having a higher peripheral
velocity than the speed of feeding said film..]..]..Iaddend.
.Iadd..[..[.32. A method as in claim 30 wherein said fiber material
is collected on a roller, said step of collecting including the
step of taking one or more layers lying on top of one another
together simultaneously from said roller..]..]..Iaddend.
.Iadd..[..[.33. A method as in claim 30 wherein the temperature in
each of said two stages is dependent upon the speed at which said
film is continuously fed, said temperature increasing with
increasing speed of film feeding and extending to a temperature
slightly lower than the melting temperature of said
film..]..]..Iaddend. .Iadd..[..[.34. A method as in claim 33
wherein said step of homopolarly charging said film includes the
step of spraying electrical charge onto at least one film surface
by corona effect and at least one grounded film support
member..]..]..Iaddend. .Iadd..[..[.35. A method as in claim 34
wherein said step of spraying includes placing at least one corona
effect device in spaced relationship to said at least one film
surface and positioning a metal grid between said at least one
corona effect device and said at least one grounded film support
member..]..]..Iaddend. .Iadd..[..[.36. A method as in claim 35
wherein said step of spraying includes applying an alternating
voltage to the corona wires of said corona effect
device..]..]..Iaddend. .Iadd..[..[.37. A method as in claim 35
wherein the step of spraying increases the spraying intensity of
the charge by positioning a metal plate a greater distance from the
film surface than said corona effect device..]..]..Iaddend.
.Iadd..[..[.38. A method as in claim 34 wherein said step of
spraying is implemented where the temperature of the film is the
highest..]..]..Iaddend. .Iadd..[..[.39. A method as in claim 38
wherein said film is stretched in stretching ratios of 1:4 and
1:1.5 in said first and second stage, respectively..]..]..Iaddend.
.Iadd..[..[.40. A method as in claim 34 wherein said step of
spraying is applied at the initial portion of said second
stage..]..]..Iaddend. .Iadd..[..[.41. A method as in claim 34
wherein said film is homopolarly charged in two stages wherein the
first charging stage corresponds to the second stretching stage and
including spraying electrical charges onto said one surface in the
first charging stage and spraying electrical charges onto the other
film surface by a corona effect with a second film support member
in the second stage of charging..]..]..Iaddend. .Iadd..[..[.42. A
method as in claim 41 wherein the electrical charges sprayed onto
the respective film surfaces are of opposite
polarity..]..]..Iaddend. .Iadd..[..[.43. A method as in claim 41
wherein charges sprayed onto each of the film surfaces are of the
same polarity and the potential applied to said second support
member is the same as the potential for spraying electrical charges
in the first charging stage and the magnitude of the potential for
spraying in the second charging stage is greater than the potential
of said second support member..]..]..Iaddend. .Iadd..[..[.44. A
method for the manufacture of an electrically charged fibrous
filter from a highly molecular non-polar material, comprising the
steps of:
continuously feeding a film of said material;
stretching said film along the longitudinal axis thereof as defined
by the path of movement of said film and including the step of
applying heat to said film;
homopolarly charging the stretched film on both surfaces thereof by
spraying one surface with a charge of one polarity and spraying the
other surface of said film with an equal charge of the opposite
polarity;
fibrillating the charged film into fiber material substantially
along said longitudinal axis; and
collecting the fiber material to form a filter..]..]..Iaddend.
.Iadd..[..[. 5. A method as in claim 41 wherein said step of
homopolarly charging is coincidental with said step of
stretching..]..]..Iaddend. .Iadd..[..[.46. A method as in claim 44
wherein each film surface is sprayed by placing a respective corona
effect device in spaced relationship to a respective film surface
and positioning a metal grid between each of said corona effect
devices and the respective film surface..]..]..Iaddend.
.Iadd..[..[.47. A method as in claim 46 wherein the step of
spraying increases the spraying intensity of the charge by
positioning a metal plate a greater distance from the film surface
than said corona effect device..]..]..Iaddend. .Iadd..[..[.48. A
method as in claim 46 wherein said step of spraying includes
applying an alternating voltage to the corona wires of each of the
cornoa effect
devices..]..]..Iaddend. 49. .Badd.An electret fibrous web having
unique filtration properties prepared by a method comprising the
steps of:
continuously feeding a film of highly molecular non-polar
material;
heating said film;
stretching said film along the longitudinal axis thereof as defined
by the path of movement of said film;
homopolarly electrically charging at least one side of said
film;
fibrillating the charged film into fiber material; and
collecting the fiber material. .Baddend. 50. .Badd.An electret
fibrous web having unique filtration properties prepared by a
method comprising the steps of:
continuously feeding a film of a highly molecular non-polar
material;
heating said film;
stretching said film;
homopolarly electrically charging at least one side of said
film;
fibrillating the charged film into fiber material substantially in
the longitudinal direction thereof as defined by the path of
feeding of the film; and
collecting the fiber material. .Baddend. 51. .Badd.An electret
fibrous web having unique filtration properties prepared by a
method comprising the steps of:
continuously feeding a film of a highly molecular non-polar
material;
heating said film;
stretching said film;
homopolarly electrically charging at least one side of said
film;
fibrillating the charged film into fiber material; and
collecting the fiber material. .Baddend. 52. .Badd.An electret
fibrous web having unique filtration properties prepared by a
method comprising the steps of:
continuously feeding a film of a highly molecular non-polar
material;
heating said film;
stretching said film;
homopolarly electrically charging at least one surface of said film
using a corona device spaced from the surface of said film whereby
the electric charge is sprayed onto the surface of said film;
fibrillating the charged material into fiber material; and
collecting the fiber material. .Baddend. 53. .Badd.An electret
fibrous web having unique filtration properties prepared by a
method comprising the steps of:
continuously feeding a film of a highly molecular non-polar
material;
stretching said film along the longitudinal axis thereof as defined
by the path of movement of said film and including the step of
applying heat to said film;
homopolarly charging the stretched film on both surfaces thereof by
spraying one surface with a charge of one polarity and spraying the
other surface of said film with an equal charge of the opposite
polarity;
fibrillating the charged film into fiber material substantially
along said longitudinal axis; and
collecting the fiber material to form a filter. .Baddend. 54.
.Badd.An electret fibrous web comprising fibers which are
fibrillated from a film and which contain electric charges embedded
therein so as to enhance filtration properties of the web.
.Baddend. 55. .Badd.A fibrous web of claim 54 in which said
fibrillated-film fibers carry opposite charges on opposite sides of
the fiber. .Baddend.
Description
BACKGROUND OF THE INVENTION
The invention relates to a method for the manufacture of an
electrically charged fibrous filter, whose fibre material consists
of a high molecular non-polar substance.
Such a method is known in the art and from this method it appears
that charging of fibre material in an electric field to obtain a
charged fibrous filter is difficult because of electric breakdown
through the pores of the material. Covering the electrodes, between
which the forming field strength is applied, with a semi-conducting
material, admittedly offers the possibility of bringing the fibre
material to a higher charged state, but at the same time has the
drawback that this state is reached only after a longer period of
time.
It is the object of the invention to provide for a rapid
manufacture of highly charged fibre filters.
SUMMARY OF THE INVENTION
According to the invention the method is characterized in that it
comprises continuously feeding a film of the high molecular
non-polar substance, stretching the film, homopolarly charging the
stretched film with the aid of corona elements, fibrillating the
stretched charged film, collecting the fibre material and
processing the collected fibre material into a filter of the
desired shape.
Because the risk of breakdown of charging a solid film material is
much less than that of an open fibre material, a charging system
known per se, operating much faster and much more effectively,
comprising corona elements can be used.
In the preferential embodiment of the invention the film is locally
bilaterally charged by means of corona elements that carry on
either side of the film equal but opposite potentials. Thereby the
film is charged to almost twice as high a voltage as by means of
unilateral charging, at one and the same corona voltage.
Charging with the aid of corona elements in turn entails that the
film can be fed continuously and be stretched into a well
splittable material. This material can be fibrillated in several
ways. For this purpose, a needle roller with metal needles running
against the film is used with, surprisingly no substantial loss of
charge.
Preferably, the fibre material is collected in layers onto a
take-up roller and there processed into filter cloth of the
thickness and shape desired by taking one or more layers, which are
laying one on top of the other, together and at the same time from
the roller.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now further be elucidated with reference to the
following drawings, wherein.
FIG. 1 schematically shows an embodiment of a device in which, for
the manufacture of a well splittable charged film, use has been
made for the method according to the invention.
FIG. 2 shows an improved second stage for stretching a film with
which the film can be provided with an injected charge on both
surfaces.
FIG. 3 shows a preferential construction of a process stage
.[.charging for improved and higher of the film.]. .Iadd.for
improved and higher charging of the film.
FIG. 4 shows on an enlarged scale an example of a set-up of
electrodes for injecting charge into the film.
In the figures like numbers refer to like elements.
FIG. 1 shows film 1, which, either from a storage roller, or direct
from an extruder is fed between roller 3 and 4 into a stretching
device to make film 1 splittable.
In this arrangement the charge is injected into film 1 from
above.
The stretching device contains fixed pins 5 and 6, block 8 heated
by heater 15, a pair of rollers 9 and 10, arcuate plate 12 heated
by heater 16 and a pair of rollers 13 and 14.
A stretching device that contains the above mentioned elements has
been described in Netherlands Patent Application 71 13047. In this
device stretching takes place in two stages. Therefore, it is very
well suitable to fibrillate films that are difficult to split.
In the first stage of stretching, which takes place between pin 6
and pair of rollers 9 and 10, film 1 is drawn over edge 7 of block
8 in such a way that film 1 is subjected to an increase in length
ratio of approximately 1 to 4 at the cost of its thickness and
hardly at the cost of its width.
In the second stage of stretching, which takes place between a pair
of rollers 9 and 10 and a pair of rollers 13 and 14, film 1 is
drawn over curved plate 12 in such a way that it is subjected to a
further increase in length ratio of approximately 1 to 1.5.
The temperature of block 8 greatly depends on the speed of the film
and at high speeds can be chosen close below the melting
temperature of film 1.
It is of importance that film 1 does not touch the plane of block 8
that lies in front of edge 7, so as to prevent a premature and a
too high heating or film 1. The position of plane 11 of block 8,
which plane lies behind edge 7 is also of importance, because it is
determinative of the speed at which film 1, coming from edge 7,
cools down.
Plate 12 is heated to a temperature that is only a little lower
than the melting temperature of the film material and because film
1 for an important part lies against the curved surface, film 1
will here receive the highest temperature in the stretching
process.
This has schematically been indicated by a triple flame 16 at plate
12, in contrast with a single flame 15 at block 8.
A charging device 18, consisting of a number of thin tungsten wires
25 across the grounded curved plate 12 and connected with the
negative terminal of a voltage source, sprays a negative charge on
to the top of film 1 by means of the corona effect. This is
implemented preferably where the temperature of film 1 on plate 12
is the highest.
It may be of advantage, however, to place the charging device more
towards the beginning of plate 12. In this case film 1 will
partially discharge over the further part of the heating plate. In
particular the charges that have been embedded in the least stable
way, will be lost in the process. In this way the electret film is
aged thermally, as a result of which only the charges are left that
have been embedded in a very stable way. As a result the remaining
charge of the electret will have an exceptionally high persistence
at ambient temperature. In fact, the thermal stability of the
charge that is left is also increased. Moreover, surprisingly, the
stability against moisture is also considerably improved.
The device furthermore shows a means 29 for the fibrillation of
film 1.
According to the embodiment film 1 is fibrillated into fibres 21 by
guiding it via fixed pin 20 along a needle roller 29. By giving the
needle roller 29 a higher peripheral velocity than the moving speed
of film 1, this film 1 is fibrillated mainly in longitudinal
direction. Fibres 21 thus obtained spread themselves to a high
extent because of their electrostatic charges, so that a nicely
spread layer of fibres is produced, which is wound upon collecting
roller 24. By taking one or more layers, which are lying one on top
of the other, from the roller together and at the same time, a
filter of the desired shape and thickness can be obtained.
From charge measurements it has been found that when negative
charges are injected on the top of film 1 on plate 12, at the
bottom positive charges are produced. These charges are a result of
ionization of the air enclosed between the film and the bottom
plate. Thus positive ions are produced in such a region, which
charges are drawn to the bottom of the negatively charged film. So,
the positive charge actually is a compensating charge. As a result,
it is somewhat less high than the injected negative charge.
This unexpected two-sided charging of film 1 can be of importance
for use in fibrous filters, because most aerosol particles that
must be captured are electrically charged, and this charge can be
positive as well as negative or both.
So as to manufacture a film that is as highly positively as
negatively charged, charging must be implemented on both sides.
FIG. 2 shows an embodiment, with which this is possible.
A second curved plate 17, which has been mounted between pairs of
rollers 9/10 and 13/14, by means of an additional charging device
19, enables the injection of positive charges on the surface of
film 1 that has not yet been sprayed upon.
On the contrary, so as to filter aerosol particles that .[.have
been.]. .Iadd.are .Iaddend.charged unipolarly, it is preferable to
apply .[.unipolarly.]. .Iadd.unipolar .Iaddend.charges of opposite
polarity to the fibre. Even for bipolarly charged aerosols the
filter mat can be composed of alternatively positively and
negatively charged fibre layers. The unipolar charging can also be
implemented by the two-stage charging of FIG. 2. Preferably, then
the potential of plate 17 is chosen the same as that of spraying
wires 25 and at spraying wires 30 a voltage is applied that is
sufficiently .[.negative.]. .Iadd.of greater magnitude
.Iaddend.with respect to plate 17.
FIG. 3 shows a preferential embodiment of a charging step with
corona elements 18, 19 on each side of the film carrying equal but
opposite potentials. This charging step 18, 19 can follow a
stretching step that is already known in the art. If, however,
charging step 18, 19 coincides with the stretching step, then
charging preferably is implemented in a furnace that is not
shown.
FIG. 4 shows one of the applied charging devices 18 and 19 on an
enlarged scale. Between spraying wires 25/30 and curved plates
12/17 over which film 1 is guided, there is a metal grid 27 so as
to better distribute the charge that is injected by the thin corona
wires. The charge the film acquires is determined by the potential
of the grid. In case of a slow throughput rate the film is roughly
charged up to the potential of the grid. An additional advantage of
the device used is that the risk of dielectric breakdown of the
film and also of a spark discharge to the bare parts of plates
12/17 is very small, because the grid screens the corona wires from
the film. Due to this grid, it is also possible to feed the small
corona wires with an alternating current instead of with a direct
current, if so desired.
Metal plate 26 over spraying wires 25/30 is interconnected with
grounded plates 12/17. Because plate 26 increases the corona
formation considerably, the spraying intensity with an upper plate
is substantially higher than in an arrangement without it.
In a simplified construction of the charging device, which gives a
somewhat less uniform charging, the grid is left out. In that case
plate 26, preferably, must be connected via terminal 32 to a
positive voltage with respect to plate 12/17. For in case of a
positive voltge on plates 26 there need not be applied a large
negative corona voltage on the small corona wires. In fact the
corona voltage can be halved, if the potential is chosen equal (but
opposite) to that of the cornoa wires. This, too, reduces the risk
of dielectric breakdown in the film considerably, particularly when
the film is very thin.
EXAMPLE I
A film of isotactic polypropylene with a thickness of 45.mu. and a
width of 5 cm was stretched to a ratio of 1:6 over block 8 the
temperature of which was 110.degree. C. In a second stage
stretching and charging was implemented over plate 12 of a
temperature of 130.degree. C., at a stretching ratio of 1:1.5. The
transport velocity was 12.2 m/min. In spraying device 18 placed
over plate 12 corona wires 25 had been connected to -3.2 KV and top
plate 26 to +3 KV. The distance from corona wires 25 to plate 12
amounted to 5 mm. The film was fibrillated with a needle roller of
60 rows, the needles of which stood 500.mu. apart. The charged
fibrillate was spread to approx. 45 cm and wound up on roller 24
into a filter with a thickness of 3 mm.
The collection efficiency of this filter and an equivalent
uncharged filter was tested with a heterodisperse NaCl aerosol at a
linear air velocity of 10 cm/sec and an aerosol concentration of 15
mg-NaCl/m.sup.3. For comparison also a commercial filter made from
glass fibres from 1 to 10.mu. was tested.
______________________________________ filter initial pressure
weight penetration loss gram/m.sup.2 % mm H.sub.2 O
______________________________________ charged filter 163 0.5 1.5
non-charged filter 163 53 2.8 filter with glass fibers 167 21 5.0
______________________________________
EXAMPLE II
The method of Example I was repeated, but the charging was now done
with the spraying device of FIG. 3, with the voltage on the corona
wires amounting to -10 KV and that on the grid to -2.3 KV. The
processing into a filter was equal to that of the above mentioned
example.
______________________________________ filter initial pressure
weight penetration loss gram/m.sup.2 % mm H.sub.2 O
______________________________________ charged filter 163 0.3 1.1
non-charged filter 163 53 2.8
______________________________________
* * * * *